As optical technology evolves to allow greater bandwidth to propagate across global networks, higher-quality and lower-cost gratings are required. Many niche applications exist where higher-quality and lower-cost gratings can make a profound difference. Some of these applications include gain flattening filters, high-powered-fiber-laser systems for directed energy weapons, wavelength lockers, dispersion compensators, and so on. In fact, fiber gratings are one of the most widely used optical components in the world today with too many applications to list here.
The transmission medium for optical technology is an optical fiber. An optical fiber is typically formed from silica glass, such as germania-doped silica glass. The inner portion of the fiber, running lengthwise, is called “the core”. The outer portion is the “cladding”. A fiber grating is a fiber having a series of crosswise lines, zones or gratings formed within it. The lines, or gratings, are narrow parallel bands having a higher refractive index than the rest of the fiber. These bands are typically evenly spaced, but need not be—depending on the desired application of the gratings.
The variations in the refractive index of fiber caused by the gratings results in a scattering of light, known as the “Bragg Effect”. In accordance with this effect, the gratings selectively reflect a narrow range of wavelengths of light. Each time the light hits a region of higher refractive index, i.e., a grating, a bit of that light is scattered backwards. If the wavelength of the light matches the spacing of the fiber grating's high refractive index zones, the light waves scattered from each high-index zone interfere constructively, producing strong reflection.
Light of other wavelengths may also propagate down the fiber. These wavelengths of light are referred to as non-resonant wavelengths. Light having non-resonant wavelengths will also be scattered by the gratings' high-index regions. But since the scattered light waves differ in phase, they will cancel each other by what is called “destructive interference”. Thus these non-resonant wavelengths are transmitted through the grating with low loss.
Fiber gratings are manufactured in many different ways and by many different companies. Although, the use of ultraviolet light and a phase mask is the most common approach for making gratings. The basics of this approach can be understood with reference to Hecht, Jeff, Understanding Fiber Optics, Prentice-Hall (2002). The phase mask is typically formed from a planar piece of silica having a pattern of fine etched parallel troughs formed therein. The pattern of troughs in the phase mask, therefore, dictates the pattern of gratings in the optical fiber.
The phase mask is disposed between the fiber and an ultraviolet light source. Regions of high and low intensity alternate within the phase mask. When ultraviolet light is applied to the mask, some portion of it passes through the troughs to the fiber to create the gratings. In regions of high intensity, the ultraviolet light creates fiber gratings by breaking atomic bonds in the fiber's core. The glass composition may be adjusted to promote the creation of better gratings.
In practice, the wavelength of importance is the wavelength in the glass, which is shorter than the wavelength in air. But the wavelength is typically characterized with respect to air, rather than with respect to the optical fiber. If D is the grating spacing and n the refractive index of the glass, the reflected wavelength measured in the air is given by:λgrating=2nDFor example, if the grating spacing is 0.500 μm and the refractive index is 1.47, the selected wavelength is 1.47 μm. In order to select a precise wavelength, the exact refractive index and grating spacing must be known. Wavelengths that do not meet the criterion established by these parameters will not be reflected in phase. That is, the scattered light waves do not add constructively. The reflected light waves average out to zero, so they are transmitted essentially unaffected. The result is a simple line-reflection filter, which reflects the selected wavelength and transmits other wavelengths.
The wavelength of light traveling through the fiber dictates how it will be reflected. Each line in a grating reflects a little bit of the light at all wavelengths. If the wavelength in the glass is exactly twice the spacing of the lines written in the fiber, all the scattered light is in phase, so the light waves interfere constructively. That wavelength is reflected. The more lines, the more uniform the spacing, and the more strongly they are written, the stronger the reflection
Variation of the reflectivity with the wavelength depends on the nature of the grating. Fine, thin, evenly spaced lines tend to concentrate reflection at a narrow range of wavelengths. Turning up exposures to make a stronger grating will increase reflectivity and broaden the range of reflected wavelengths. Commercial devices using this design select a range of wavelengths as narrow as a few tenths of a nanometer and ranging up to several nanometers wide. The narrow ranges are well matched to the requirements of wavelength-division multiplexing in the 1.55 μm band.
A variety of factors can effect the refractive index achieved by the gratings. For instance, the extent of ultraviolet irradiation, the glass composition, and any special processing before treatment can each effect the refractive index of the grating. Typical processes for forming gratings expose the fiber to high intensity pulsed ultraviolet lasers for a few minutes. This can increase the refractive index of, for example, germania-doped silica by a factor of 0.00001 to 0.001. It is known that treating the fiber with hydrogen before illuminating it with ultraviolet can increase the photosensitivity, so the refractive index increases up to 1%. This is referred to as hydrogen loading of the fiber. The higher levels of change are comparable to the difference in refractive index between the core and the cladding, which typically does not reach 1% in a single mode fiber.
Many companies have spent millions of dollars in research attempting to improve the performance of gratings, and the processes for creating them. Some companies have introduced expensive dopants into the glass during the fiber fabrication process in an attempt to get a fiber with more photosensitivity and, therefore, a better grating. These doped gratings are very expensive to manufacture, but are popular in the industry due to the performance benefits compared to other existing methods and the lack, to date, of better alternatives.
It would be advantageous to have a system and method for increasing the photosensitivity of an optical fiber without the high cost of doping the fiber.